Because of its clean burning qualities and convenience, natural gas has become widely used in recent years. Many sources of natural gas are located in remote areas, great distances from any commercial markets for the gas. Sometimes a pipeline is available for transporting produced natural gas to a commercial market. When pipeline transportation is not feasible, produced natural gas is often processed into liquefied natural gas (which is called "LNG") for transport to market.
One of the distinguishing features of a LNG plant is the large capital investment required for the plant. The equipment used to liquefy natural gas is generally quite expensive. The liquefaction plant is made up of several basic systems, including gas treatment to remove impurities, liquefaction, refrigeration, power facilities, and storage and ship loading facilities.
LNG refrigeration systems are expensive because so much refrigeration is needed to liquefy natural gas. A typical natural gas stream enters a LNG plant at pressures from about 4,830 kPa (700 psia) to about 7,600 kPa (1,100 psia) and temperatures from about 20.degree. C. (68.degree. F.) to about 40.degree. C. (104.degree. F.). Natural gas compositions at atmospheric pressure will typically liquefy in the temperature range between about -165.degree. C. (-265.degree. F.) and -155.degree. C. (-247.degree. F.). This significant reduction in temperature requires substantial refrigeration duty.
It has been recently proposed to transport natural gas at temperatures above -112.degree. C. (-170.degree. F.) and at pressures sufficient for the liquid to be at or below its bubble point temperature. For most natural gas compositions, the pressure of the natural gas at temperatures above -112.degree. C. (-170.degree. F.) will be between about 1,380 kPa (200 psia) and about 4,500 kPa (650 psia). This pressurized liquid natural gas is referred to as PLNG to distinguish it from LNG, which is transported at near atmospheric pressure and at a temperature of about -162.degree. C. (-260.degree. F.). The production of PLNG requires significantly less refrigeration than that required for the production of LNG since PLNG can be more than 50.degree. C. warmer than conventional LNG at atmospheric pressure. Examples of processes for manufacturing PLNG are disclosed in U.S. patent applications Ser. No. 09/099262, 09/099590, and 09/099589 and in U.S. provisional application No. 60/079642. In view of the substantial economic benefits associated with making and transporting PLNG, a continuing need exists for improved processes for producing PLNG.